1. Field of the Invention
This invention relates generally to telemetry systems for MWD well logging systems.
2. Description of the Prior Art
Data about underground strata being penetrated by a drill bit is valuable to well drillers, particularly if the data can be obtained during drilling operations without having to remove the drill string. However, the down-hole environment is extremely harsh including elevated temperatures, high pressures, and severe vibrations. Drilling fluids are also highly abrasive.
Systems capable of providing such real time data are typically referred to as, "MWD" or Measure-While-Drilling well logging tools and are serially incorporated into a drill string above the drill bit. A data telemetry or telecommunications link capable of transmitting data from down-hole to ground surface instruments and capable of transmitting control signals from ground surface to down-hole instruments are an essential subsystem in any MWD well logging tool.
In U.S. Pat. Nos. 2,354,887, 2,389,241, and 2,411,696, Silverman describes proposed MWD systems for collecting and transmitting data utilizing the drill string and either toroidal transformers or insulated electrodes to induce electromagnetic currents in underground formations for both data collection and "well signaling." In U.S. Pat. Nos. 3,793,632 and 4,302,757, Still discloses methods utilizing the drill string for data transmission with low frequency electromagnetic waves induced by toroidal transformers (virtual electrodes). Other electromagnetic wave transmission systems for establishing a data/control telemetry link between ground surface and down-hole instrumentation are described in U.S. Pat. Nos. 4,181,014, Zuvela et al; 4,087,781, Grossi et al; 4,057,781, Scherbatskoy; and 3,967,201, Rorden.
The theoretical basis for utilizing a drill string as an element for inducing electromagnetic waves or currents underground has been presented by J. R. Wait and D. A. Hill, in an article entitled "Theory of Transmission of Electromagnetic Waves Along a Drill Rod in Conducting Rock" IEEE Trans. on Geoscience Electronics, Vol. GE-17, No. 2, (5/79) pp. 21-24); by G. S. Smith & R. W. P. King, in an article entitled "Resonant Linear Antenna as a Probe for Measuring the In-Situ Electrical Properties Of Geological Media" J. of Geophysical Research, Vol. 79, No. 17, (6/74) pp. 2623-2628; and by K. Lee & G. Smith, in an article entitled "Measured Properties of Bare and Insulated Antennas in Sand" IEEE Trans. on Antennas & Propagation, Vol. AP-23, No. 5, (9/75) pp. 664-670.
Many different types of electromagnetic wave (EM) telemetry systems have been proposed, postulated and even tried for MWD logging tools. From the literature it is apparent that many of such EM telemetry systems would work if the underground strata were homogeneous, and undifferentiated.
However, the strata underground is typically quite inhomogeneous and differentiated. Each type of geological strata has unique properties which greatly affect transmission of electrical energy/signals through it and which vary depending on depth, local and the sequence and spacing of the different formations, among other factors. Also the electrical properties of the drill string dynamically vary with many factors including length, drilling fluid and temperature.
In summary, an EM data/control telemetry system linking an MWD logging tool to ground surface must not only survive elevated temperautres, high pressures, severe vibrations and abrasion, it must also be able to adapt and function in a constantly varying electrical environment.